The production of photopolymer relief images for flexographic printing is generally carried out by imagewise exposing the photosensitive layer of a flexographic printing plate precursor using suitable imaging radiation such as UV radiation. Unexposed areas of the photosensitive layer are washed off (developed or “washed out”) using a suitable developer or processing solution while exposed, crosslinked areas are left intact. Residual developer is generally removed by evaporation and if necessary, the developed surface is treated to remove tackiness. The flexographic printing plate is usually wrapped around a cylinder on a printing press and used to transfer ink to a suitable substrate such as papers, films, fabrics, ceramics, and other materials.
While the non-exposed photopolymer may be soluble in a variety of organic solutions, only some of those solutions are the best developers that do not damage or swell the crosslinked portions while cleanly removing the non-crosslinked portions. Swelling will eventually cause the relief image to deteriorate in the processing bath and the processing solution will have to be changed frequently due to the build-up of sludge.
Suitable developers have generally comprised organic solvents such as saturated hydrocarbons, aromatic hydrocarbons, aliphatic ketones, terpenes, and chlorinated hydrocarbons such as trichloroethylene, or mixtures of such solvents that may also include lower molecular weight alcohols depending upon the polymer binders used in the photosensitive layer.
Developers have been carefully formulated as mixtures of organic solvents to overcome various problems such as acting too slowly so that plate swelling occurs, solvent toxicity, safety concerns, long drying times, and ineffective removal from the developed printing plate. For example, U.S. Pat. No. 5,354,645 (Schober et al.) describes developers for flexographic printing relief images that include one or more of diethylene glycol dialkyl ethers, acetic acid esters or alcohols, carboxylic acid esters, and esters of alkoxy substituted carboxylic acids.
The development or washout of imaged photopolymerizable flexographic printing plate precursors is also described, for example, in U.S. Pat. Nos. 6,162,593 (Wyatt et al) and 5,248,502 (Eklund) and U.S. Patent Application Publication 2005/0227182 (Ali et al.), and the references cited in therein. Representative processing solutions (developers) are also described in U.S. Pat. No. 6,248,502 (Eklund). The photopolymer compositions used in the flexographic printing plate precursors are generally carefully matched to the desired processing solutions.
For example, U.S. Pat. No. 6,582,886 (Hendrickson et al.) cites a number of publications that describe various “matched” photosensitive layers and developers. This patent specifically describes developers containing methyl esters alone or mixtures of methyl esters and co-solvents such as various alcohols that are soluble in the methyl ester(s). These developers can also include various non-solvents such as petroleum distillates naphthas, paraffinic solvents, and mineral oils.
Other known processing solutions include chlorohydrocarbons, saturated cyclic or acyclic hydrocarbons, aromatic hydrocarbons, lower aliphatic ketones, or terpene hydrocarbons. While these processing solutions are effective for the intended purpose, they have a number of disadvantages. They may act too slowly, causing swelling in the printing plates and thus damage the fine detail in the relief images, require long drying times, and have low flashpoints resulting in dangerous situations. Many of these solvents are also considered hazardous air pollutants (HAPS) and are subject to stringent governmental reporting requirements. They may also be too toxic for direct disposal into the environment or create worker safety problems with strong odors or handling problems.
Copending and commonly assigned U.S. Ser. No. 12/545,268 filed Aug. 21, 2009 by Bradford and based on Provisional Application 61/097,358 (filed Sep. 19, 2008 by Bradford) describes flexographic printing plate washout or processing solutions containing dipropylene glycol dimethyl ether (DME) alone or in combination with various co-solvents such as alcohols and aliphatic dibasic acid ethers.
U.S. Pat. No. 6,162,593 (noted above) describes the use of diisopropylbenzene (DIPB) alone or with alcohol co-solvents in processing solutions.
Whether the processing solutions are reclaimed or not, there is a need for more environmentally friendly developing solutions that offer improvements in handling, disposal, low odor, and low levels of volatility (lower level of “volatile organic chemicals”, or “VOC” content) while still providing the effective cleanout (or washout) of non-polymerized material in an imaged flexographic printing plate precursor. It is also desirable that the processing solution be useful for a variety of photopolymer compositions so that the processing solution is versatile for multiple uses.
While many of the known developers are effective for removing non-crosslinked materials in the imaged flexographic printing plate precursor, the developers are not necessarily environmentally friendly or free of health hazards (toxicity). Thus, there continues to be a need to provide improved flexographic printing plate developers that are non-hazardous, non-flammable, and environmentally friendly especially to aquatic life, and have low odor and a reduced level of VOC.